A truck drives through a flooded street, caused by Hurricane Sandy, on October 30, 2012, in the Lower East Side of New York City. (Andrew Burton/Getty Images)

Gary Strauss, Kevin McCoy and Carolyn Pesce, USA Today

After leaving a deadly trail of devastation in New Jersey and New York, Superstorm Sandy's brute power is fading as it courses north. But the threat of heavy rains, snow and continued flooding could linger over a huge swath of the Northeast and Midwest for several days.

Now classified as an "extratropical" storm by the National Weather Service, Sandy has caused at least 50 U.S. deaths so far - 25 in New York, including 18 in flood- and wind-ravaged New York City.

An estimated 8.2 million people were without power Tuesday across 17 states and the District of Columbia. About 2.3 million are in New Jersey alone, where Gov. Chris Christie said Sandy left "absolute devastation" and "unthinkable damage" along coastal towns ravaged by flooding and gale-force winds. More than 20,000 residents of Hoboken were reported stranded late Tuesday night as flooding conditions worsened along the Hudson River.

President Obama is expected to survey damage in hard-hit areas of New Jersey Wednesday. "We're going to do everything to help you get back on your feet," he said.

Although Sandy's wrath had prompted the cancelation of some 16,000 airline flights through Tuesday, some limited air travel is expected to return to the New York City metro area Wednesday .The Port Authority of New York and New Jersey says John F. Kennedy International Airport in New York and Newark International Airport in New Jersey will open at 7 a.m. with limited service.

The massive, pinwheel-shaped storm, nearly 900 miles wide, is expected to drub the upper Northeast and Midwest as it moves into Canada this afternoon.

Flooding remains a concern in several states, while bone-chilling temperatures and the prospect of heavy snow remains problematic in the Appalachians, where up to 3 feet has already fallen in parts of Maryland and West Virginia. Wind gusts of up to 60 mph were pummeling Lake Michigan on Chicago's lakefront. In a measure of its massive size, waves on southern Lake Michigan rose to a record-tying 20.3 feet. High winds spinning off Sandy's edges clobbered the Cleveland area Tuesday, uprooting trees, closing schools and flooding major roads along Lake Erie.

Restoring power to New York City's boroughs remains a challenge and could take a week or longer.

"Recovery is a mammoth job" in the days ahead, said New York Mayor Michael Bloomberg, who called Sandy among the worst to ever hit the nation's most populous city.

Few weather events inflict more damage on electrical substation equipment than floods, says Nicholas Abi-Samra, an expert on the effect of extreme weather on power grids. "Restoring flooded substations takes much longer than restoring a downed power line," says Abi-Samra, a manager with North Carolina-based energy consultant Quanta Technology.

More than 6,000 New York City residents are still in the city's 76 evacuation centers. Some 110 homes were destroyed or heavily damaged by wind-swept flames in the tiny Queens borough beachfront community of Breezy Point. "We're all devastated. But we're so thankful that this time no one was killed," said Marilyn Coady, a resident for 46 years who lives near the six-block area that was destroyed in the storm.

In New Jersey, Gov. Christie described devastation including seaside rail lines washed away and parts of the coast still underwater.

"It is beyond anything I thought I'd ever see," he said. "It is a devastating sight right now."

Bloomberg said New York City schools would remain closed Wednesday but city workers would return to their jobs. Evacuation orders remain in place for low-lying areas of the city.

The storm sent a surge of water over seawalls in Lower Manhattan and into streets, subway stations and electrical equipment. A large tanker ran aground on Staten Island and winds collapsed a construction crane 74 stories high atop an expensive new condo building. Bloomberg said people evacuated from around the crane cannot return until it is stabilized.

Sandy will end up causing about $20 billion in property damage and $10 billion to $30 billion more in lost business, making it one of the costliest natural disasters on record in the U.S., according to IHS Global Insight, a forecasting firm.

Bloomberg, who said he expects the city's death toll to rise as emergency workers move through neighborhoods that were among the hardest hit, gave a somber account of some of those who died.

They included two who drowned in a home and one who was in bed when a tree fell through an apartment. A 23-year-old woman died by stepping into a puddle near a live electrical wire. A man and a woman were crushed by a falling tree.

An off-duty officer on Staten Island who ushered his relatives to the attic of his home apparently became trapped in the basement.

Sandy also killed 69 people in the Caribbean before making its way up the Eastern Seaboard.

Obama declared New York and New Jersey federal disaster areas.

The disaster declaration makes federal funding available to residents and businesses in the affected areas, which bore the brunt of the sea surge from the superstorm. Speaking during a stop Tuesday at Red Cross headquarters, Obama warned that the massive storm "is not yet over."
He said there were still risks of flooding and downed power lines and called the storm "heartbreaking for the nation."

The president said he told governors in affected areas that if they get no for an answer, "they can call me personally at the White House."

Jeff Masters, meteorology director for Weather Underground, a private forecasting service, called the storm surges some of the highest ever recorded.

In New Jersey, where the storm came ashore, hundreds of people were evacuated in rising water Tuesday. Officials used boats to try to rescue about 800 people living in a trailer park in Moonachie.

A hoarse-voiced Christie gave bleak news at a morning news conference: Seaside rail lines washed away. No safe place on the state's barrier islands for him to land. Parts of the coast still under water.

"It was an extremely devastating and destructive storm, hopefully one that people will only see once in their lifetime," said Joe

Pollina, a National Weather Service meteorologist.

Con Edison spokeswoman Sara Banda said power was out for 804,000 of their customers in New York -- four times the number affected by Hurricane Irene. "This is the largest storm-related outage in history," Banda said.

New York Gov. Andrew Cuomo in an interview with WCBS News radio said Tuesday that "power restoration is going to be a real challenge."

"You want to talk about a situation that gets old very quickly. You are sitting in a house with no power and you can't open the refrigerator," he said. "That gets very frustrating."

A massive explosion at a power substation in Lower Manhattan on Monday evening contributed to the power outages. No one was injured, and the power company did not know whether the explosion was caused by flooding or by flying debris.

New York University's Tisch Hospital was forced to evacuate 200 patients after its backup generator failed. NYU Medical Dean Robert Grossman said patients - among them 20 babies from neonatal intensive care that were on battery-powered respirators - had to be carried down staircases and to dozens of waiting ambulances.

Stock trading was closed in the U.S. for a second day Tuesday - the first time the New York Stock Exchange will be closed for two consecutive days due to weather since 1888, when a blizzard struck the city. Trading was scheduled to resume on Wednesday.

Tonight's Halloween Parade in Greenwich Village has been postponed until some time next week, Bloomberg said. The city's famed New York City Marathon is still scheduled for Nov. 4.

Sandy is still expected to produce strong winds across the Mid-Atlantic and New England, as well as rainfall amounts of 4-8 inches over portions of the Mid-Atlantic. Additionally, snowfall totals of 2-3 feet were possible in the mountains of West Virginia, where blizzard warnings remain in effect.